Conformal coatings are protective coatings that conform to the surface of a substrate or article (for example, dielectric coatings on electronic components and circuit board assemblies). Properly applied conformal coatings can increase the working life of a circuit board assembly by protecting its components and the board itself. Conformal coatings can, for example, provide a barrier to moisture, chemicals, dust, fungus, ultraviolet light, and ozone, as well as act as a stress-relieving shock and vibration absorber.
Various materials such as, for example, polyurethanes, acrylics, epoxies, and silicones are commonly used for conformal coatings. The selection of a conformal coating material is generally based upon desired performance and processing requirements for a specific application. Silicones, for example, are known for high temperature operating capability and can typically be used at operating temperatures up to about 200° C., whereas many other conformal coating materials can typically only be used at operating temperatures up to about 130° C. Silicones, however, exhibit a high coefficient of thermal expansion and relatively poor adhesion to many materials. Acrylics, on the other hand, are generally the easiest of the coating materials to handle and rework, and typically provide better adhesion properties than silicone. Acrylics, however, often suffer from the temperature limitations mentioned above.
Another consideration in the selection of a conformal coating material is the cure mechanism required by the material. Depending upon the type of material, the conformal coating may be a heat cure, ultraviolet (UV) light cure, moisture cure, or no cure system. The substrate or article being coated must be capable of withstanding the cure mechanism required by the material. Therefore, conformal coating materials that require no cure mechanism are typically most desirable.
Proper coverage and uniformity of the conformal coating over the assembly is critical for effective protection of the assembly. It can sometimes be difficult, however, to determine the integrity and uniformity of the conformal coating when it is coated on an assembly. Therefore, the conformal coating material is sometimes doped with a dye or “tracer” to aid in the quality assurance inspection of the assembly for proper coverage. Fluorescent tracers work well in many conformal coating materials, but the use of fluorescent dyes in fluoropolymers is limited because of poor compatibility of the dyes with the polymer matrix. This results in poor color quality and dye bleed and little or no color entrainment into the resulting coating.